1. Field of the Invention
This invention relates generally to television aperture masks and, more specifically, to a process for forming openings of various sizes in a television aperture mask.
2. Description of the Prior Art
The concept of aperture masks for television picture tubes is well known in the art. A typical prior art aperture mask is shown in the Braham U.S. Pat. No. 2,750,524 which shows an aperture mask having a plurality of circular openings.
The operation of such aperture masks in a television picture tube may be found in the Fyler et al U.S. Pat. No. 2,690,518 which shows a color television tube having an aperture mask located as an electron beam screen.
The prior art aperture mask openings have taken many different shapes including round as shown in the aforementioned patents or elongated as shown in the Suzuki et al U.S. Pat. No. 3,883,347. While the shape of the opening may vary in different masks, generally, all masks require the open area in the aperture mask to be graduated to accommodate the characteristics of the human eye. That is, if a television picture is to appear uniform in brightness to the human eye, it is necessary to have a television picture where the central area of the television picture is actually brighter than the peripheral area. To obtain a brighter central area the aperture masks are usually made with larger size openings in the center of the mask and smaller size openings in the periphery of the mask with openings of intermediate sizes located therebetween. As the brightness of a television picture tube is directly proportional to the open area of the aperture mask, the use of a constant density of apertures with gradually decreasing size produces an image that appears uniform in brightness to the human eye. Typically, if the brightness or open area is a maximum of 100% in the center of the aperture mask, it decreases to a minimum of 70% in the peripheral region of the aperture mask. The prior art Tsuneta et al U.S. Pat. No. 3,652,895 shows an aperture mask having a plurality of rectangular slots or circular openings with the size and pitch of the openings decreasing in size from the center of the mask to the peripheral portion of the mask. FIG. 13 of the Tsuneta et al patent also shows an alternate concept in which instead of varying the aperture size, the space between apertures is increased to thereby decrease the open area on the peripheral regions of the mask.
While the concept of decreasing open area from the center of the aperture mask to the periphery of the aperture mask is well known, the method of making an aperture mask with various size openings in which the openings are within proper tolerances has been quite difficult. The prior art Tsuneta describes the use of multiple pattern carriers that are superimposed to form a graduated pattern that is transferred onto the light-sensitive coating located on the surface of a sheet of aperture mask steel. The Tsuneta method employs the characteristics of a light source with radially decreasing intensity to develop a light-sensitive film so that the open areas in the light-sensitive film decrease radially outward.
Still another method of decreasing the size of the openings in an aperture mask is taught in the Frantzen et al U.S. Pat. No. 3,788,912. Frantzen et al teaches the nozzle position and the amount of spray can be varied to provide larger or smaller openings in selected regions of the mask. In the Frantzen technique the openings in the photoresist are of equal dimensions throughout the aperture mask with control of the aperture size obtained through controlling the etchant supply. Typical aperture masks in use today are made from a base material and have a cone side surface and a grade side surface. The cone side surface comprises a set of hollowed out recess region located on one side of the aperture mask. Located in the hollowed out recess region is an elongated or circular aperture.
To etch aperture masks with a cone side and a grade side wherein the photoresist pattern remains constant throughout the surface of the aperture mask, it is oftentimes necessary to vary also the time of etching as well as the spray direction and the amount of etchant sprayed on the aperture mask. To vary the spray time in mass production lines requires a series of multiple etching stations such as shown in Frantzen U.S. Pat. No. 3,788,912 which the number of etching stations used to determine the total etching time. However, such techniques are difficult to use and depend a great deal on the skill of the operator.
The process of the present invention in contrast eliminates the dependence on the skill of the operator by defining the openings in one side of the photoresist according to a parameter hereinafter referred to as the over-etch factor.